Protective devices



March 13, 1956 Filed June 29, 1953 N. BATTLE ET AL PROTECTIVE DEVICES 5 Sheets-Sheet l if 1% if if P .B. SM/T/l N. BATTLE ET AL PROTECTIVE DEVICES Filed June 29, 1953 5 Sheets-Sheet 2 March 13, 1956 N. BATTLE ET AL PROTECTIVE DEVICES 5 Sheets-Sheet 3 Filed June 29, 1955 WN I I N. BATTLE ET AL 2,738,029

PROTECTIVE DEVICES 5 Sheets-Sheet 4 v w i:!- mm Nu QR WWW o B B o P. J, *W 4 AN v x 5: ii o a R \w \w o wh o 0 Q Wm o o o QM Q wsww M \w Sq March 13, 1956 Filed June 29, 1953 N. BATTLE ET AL PROTECTIVE DEVICES March 13, 1956 5 Sheets-Sheet 5 Filed June 29, 1953 Kg? VIA/TIPS 0 Barruw P .B. 9M1? ll United States Patent 0 PROTECTIVE DEVICES Norman Battle, Chilwell, and Philip Burrows Smith,

Mickleover, England, assignors to Rolls-Royce Limited, Derby, England, a British company Application June 29, 1953, Serial No. 364,790 Claims priority, application Great Britain July 4, 1952 22 Claims. (Cl.- 18362) This invention relates to protective devices such, for example, as are employed in the air intake to a gas-turbine engine for preventing debris entrained in the airfl'ow in the air intake being carried into the engine and thereby causing damage.

More specifically this invention is concerned with debris guard arrangements of the kind (hereinafter referred to as the kind specified) having a guard comprising a grid portion which is arranged to be retractable from an operative position in which the grid portion is arranged, either alone or together with another grid portion or other grid portions which may also be similarly adjustable, to extend across the whole of the path of. a

fluid flowing in a duct, for instance an air intake, thereby to catch debris entrained in the fluid, to an inoperative position in which the portion is out of the fluid path in the duct, for instance in which it is accommodated" in a recess in a wall of an air intake duct, and having also power means to move the grid portion between its operative and inoperative positions.

In the following description reference will be made only to the use of debris guard arrangements of the kind specified applied to air intakes of gas turbine engines, but it will be understood that the invention may be" applied to other uses.

In the application of a debris guard arrangement of the kind specified to an annular air intake duct, 'there may be provided a number of grid portions each formed substantially as a sector of the annulus and arranged to swing. about circumferentially-extending axes adjacent the outer edgesof the respective grid portions so thatthe grid: portions in their inoperative positions are; a'ccom modat ed within an outer wall of the air intake duct.

Moreover, the grid portions may be arranged to swing forwards in moving from. their operative positions to their inoperative positions so that any debris on: a grid-portion is carried on the portion to outside the air path through the air intake duct; conveniently the grid portions are Qllljved to conform with. the outer wallof theduct when in their retracted positions; and in the case where the grid sections swing forwardly the grid sections may in their operative position lie approximately on the surface. of a fru stum of a cone, the narrower end of which is upstream of the larger end, and in this case the diameter of the smaller end of. the frustum will be-substantiallyequal to the inner diameter of the annular duct.

The present invention has for an object to provide an improved debris guard arrangement of the kind specified in which the power required to move the grid portion or portions between the operative and inoperative positions is small as compared with. known arrangements in which the grid sections swing about axes. adjacent an edge of the gridlportion.

According to the present invention in a debris; guard arrangement of the kind specified, a grid portion is supported to move between its operative and inoperative positions so that in each position of the grid portion the line of action of the load on the grid portion dueto the 2,738,029 Patented Mar. 13, 1956 airflow in the air intake duct passes through or close to the corresponding instantaneous centre of rotation of the grid section, at least over the major part of the travel of the grid portion between its operative and inoperative positions. With the arrangement of the invention, the moment of the load due to the airflow in the air intake duct opposing turning of the grid portion about, its instantaneous centre of rotation can be kept small or negligible so that the operating load required is substantially reduced. It will be appreciated that if the] load opposes movement of the grid portion during retraction, itwill assist movement towards the operative position and vice versa. The grid portion may be so supported that over part of the movement between its operative and inoperative positions, the line of action of the load lies to one side of the instantaneous centre of rotation, and that over another part of the movement the line of action lies to the other side of the instantaneous centre of rotation, or the grid portion may be so supported that, say, the line of action of the resultant load passes at each instant through the instantaneous centre of rotation.

According to one arrangement of this invention, the power means is connected to the grid portion so that the point of connection of the power means to the grid portion moves in a straight line substantially parallel to the direction of airflow through the air intake, and the grid portion is supported so that the locus of the centre of pressure of the grid portion is substantially a straight line at right angles to the linejof movement of the point of connection of the power means to the grid portion. It can be shown that by so arranging the movement of the centre of pressure of the grid portion in relation to the line ofmovernent of its point of connection to the power means, and by arranging that the line of action of the load due to the airflow passes through the instantaneous centre of rotation of the grid portion when the grid portion is in the "operative position, the line of action of the load due to the airflow in the air intake at each instant will pass through or close to the instantaneous centre of rotation, at least over the'major part of the travel of the grid portion between its operative and inoperative positions.

It will be appreciated that in accordance with this feature of the invention, substantially no work is done against the air load in moving the grid portion from the operative to the retracted position, or vice versa.

In one construction according to this arrangement of the invention, a grid portion is supported by a link or links pivoted to the grid portion and to the fixed structure, and

- the grid portion is movedbetween its operative and inoperative positions by a ram connected to it at its edge, and it is arranged that the points of connection of each link and ram to the grid portion and to the fixed structure are such that, when a grid portion is viewed along its instantaneous axis of rotation, itslink or links appear to have a length equal to half the distance between the centre of pressure of the grid portion and the point of connection to it of the ram, the link or links appear to be connected to the grid portion midway between its centre of pressure and its point of connection to the ram, and the link or links appear to be connected to the fixed structure on the line of travel of the point of connection of the grid portion to the ram.

With such an arrangement the centre of pressure will travel along a line which is at right angles to the line of travel of the pointof connection of the grid portion to the ram, and the line of action of the load due to the airflow in the air intake duct will at each instant pass through the instantaneous centre of rotation of the grid portion.

With this arrangement however, if the grid portion is arranged to swing forwardly, as is desirable, the points of; connection of the link or links to the fixedstructure stantially parallel to the direction of airfiow through the air intake duct, but in which the link or links by which the grid portion is connected to the fixed structure are pivoted to the fixed structure on the downstream side of the grid portion inwardly of the point of connection of the ram to the grid portion and are pivoted to the grid portion at points oflset on the downstream side of the line joining the point of connection of the link to the fixed structure and the point of connection of the ram to the grid portion and radially between these points.

It will be appreciated that when the grid portions are formed as sectors in an annular duct, the pivot point on the fixed structure may comply with the geometrical configuration described and yet may lie outside the outer diameter of the duct.

By suitable dimensioning of the length of the link or links and by suitable spacing of the points of connection of the links and ram to the stationary structure and grid portion, it can be arranged that the centre of pressure of the grid portion moves in a direction substantially at right angles to the line of travel of the point of connection of the ram to the grid portion, that in the operative position of the grid portion the line of action of the load on the grid portion due to the airflow passes through the centre of rotation, that after the initial retracting movement the line of action lies close to the centre of rotation and on the side of it such that the moment due to the load on the grid portion opposes movement thereof to its inoperative position, and that as the grid portion approaches its inoperative position the line of action of the load is close to the instantaneous centre of rotation but on the opposite side of it. With this arrangement the grid portion is initially balanced so that no load (other than that required to overcome friction and the weight of the grid portion) is required for the initial retracting movement, and as it approaches its inoperative position the load due to the airflow tends to assist the final movement of the grid portion to its inoperative position; also the load due to airflow tends to assist in the final movement of the grid portion from its inoperative position towards its operative position.

In a slightly modified arrangement, the line of action does not pass through the centre of rotation when the grid portion is in its operative position, but lies to the side of it so that the grid is unbalanced in the operative position and the load on the grid tends to assist the final movement of the grid portion to its operative position, and to hold it in that position.

In a third arrangement the same difficulty of the first arrangement is avoided and the use of supporting links is also avoided. In this arrangement the power means again comprises a ram device connected to the grid portion adjacent its edge and arranged so that the point of connection of the ram to the grid portion moves along a straight line substantially parallel to the direction of airflow through the air intake; in this construction, however,

a second point of the grid portion adjacent the same edge,

but offset to the downstream side of the point of connection of the ram with the grid portion, and inwardly thereof, is constrained to move along a rectilinear path which, as the grid portion moves from its operative position to its inoperative position, diverges from the path of the corresponding point of connection of theram to the grid portion towards the axis of the air intake. It will be appreciated that When the grid portions are formed as sectors in an annular duct, the pivot point on the fixed structure may comply with the geometric configuration described and yet may lie outside the outer diameter of the duct. The effect of the airflow on the ram load is similar to that for the second arrangement, except that in the operative position the airflow tends to oppose movement of the grid towards the retracted position, and after about one third of its movement towards the retracted position the grid portion passes through the balanced condition so that as it moves further towards the inoperative position the load due to airflow tends to assist the final movement to its inoperative position.

Preferably in each arrangement rollers or other antifriction means are provided on the grid sections at their points of connection to the rams, at the points of connection of the links to the grid sections, and at the points of the grid sections constrained to move in divergent rectilinear paths, and the rollers are arranged to be guided in the fixed structure, for instance by running on the flanges of a channel section or in tubes slotted lengthwise to permit the roller spindles to project out from the tubes.

A number of grid portions mounted for retraction in the manner described may be provided where the air intake duct is of annular form. Preferably a separate ram is provided for each grid portion, or two rams may be used for each grid portion, spaced apart circumferentially. A pair of circumferentially-spaced links will normally be provided for each grid portion.

In each construction the grid portions are preferably formed from metal slats arranged parallel to one another, and where a number of grid portions are employed in an annular intake duct, one of the slats adjacent the centre of the grid portion as viewed axially of the intake is arranged to be radial with respect to the axis of the intake duct, and the frames of the grid portions may such that the adjacent edges of two adjacent grid portions are either radial or inclined to radii drawn through their inner ends. The slats may be connected together and stiffened by arcuate or chordwise extending strip members, the ends of which are attached to the side members of the frames. Alternatively, the grid portions may be formed by a series of parallel slots of which one is radial and a corresponding series of arcuate or chordwise extending slats. The slats may be of aerofoil section.

In each of the above arrangements the rams may be connected either one to each grid portion midway along the edge on the side towards which the grid portion is retracted, or one at each of the circumferentially-spaced ends of the edge, and the guide rollers which are constrained to move on the divergent paths and the links are preferably arranged adjacent the circumferentiallyspaced ends of this edge.

There will now be described with reference to the accompanying drawings and in more detail, the three arrangements above set forth.

In the drawings:

Figure 1 illustrates the first arrangement,

Figure 1A is a graphical illustration of the effect of the arrangement of Figure 1,

Figure 2 illustrates the second arrangement,

Figure 2A corresponds to Figure 1A and illustrates the effect of the arrangement of Figure 2,

Figure 3 illustrates the third arrangement,

Figure 3A corresponds to Figure 1A and illustrates the effect of the arrangement of Figure 3,

Figure 4 is a view partly in section of a practical form of the arrangement of Figure 2,

Figure 5 is a section on the line 5-5 of Figure 4,

Figure 6 is an axial view of part of Figures 4 and 5 and having parts broken away,

Figure 7 is a side elevation of the part shown in Figure 6,

Figure 8 is a detail view, and

i of arrow 9.

Figure 9 is an end view of Figure Binthe direction In each of Figures l, 2 and 3, the guard grid is. indicated at 10 and is shown in the operative position. In each of these figures the operating ramis. shown at 11 and its piston 11a is shown to be connected to the guard grid 10. at a point 12 carrying a roller 13 which is constrained to move in guides 14 parallel to the air flow through the air intake duct, the direction of air flow being indicated by an arrow 15; In each, figure, moreover, the centre of pressure of, the gnard grid is indicated at 16.

Referring now to Figure 1., 17 indicates al-ink by which the grid portion 10 is connected tofixed structure and it will be seen that the point 18 at which the link 17 is, pivotally connected to thegl'idportion 10v is halfway between the point 16 and the point 12, that the length of the link 17 is half the distance between thepoint. 16 and the point 12, and that thepoint19 by which. the link, is

connected infixed structure is on the lineof travel'12a of the point 12. as guided by the guides 14. The link 17 and pivots 18, 19 provide a support including guide means for the pivot. 18 such that the line of travel of'the pivot axis is in a path which is at an angle to the path of the point 12.

Referring now to Figure 2, 20 indicates a. link by which this 'grid portion 10 is supported from the fixed structure and it will be seen that the point 21 by which the link ispivotally connected to fixed structure is radially inwards of the point 12 and. is on the downstream:

side of the grid portion 10, and. that the; point 22. by which the: link is connected to the grid portion 10 is radially between the points 12 and 21 and offset from the line joining them on the downstream side of the line. The

link 20 and pivots 21, .22v provide a. support including guide means for the pivot 22 such that the line of travel of pivot 22 is in a path which isat anangle.- to the path of the point 12.

Referring now to Figure 3, 23 indicates a roller carried y the grid. portion on a lug 24'to turn about, an axis 25 which is on the downstream side of: the grid section 10 and. inwards of the point 12, and the roller 23 is constrained by guides 26 to. cause the point 25 to move along a line 25a which diverges from the line 12a in the direction. of movement of the point 25when the grid portion 10 is moved from its operative position.

Referring now to Figures 1A, 2A, 3A, in each case a the thick trace R. indicates the travel of the point 12, the

thick. trace S indicates the locusof. the corresponding instantaneous centre of rotation, and the thick. trace T in- .dieates. the locus of the centre of. pressure 16; Ineach case, points 1, 2, 3' and 4. on the traces. indicatecor respending, positions, point 1 corresponding to the operative position and point 4 'to the retracted position, and it will be: seen that:

i (a) In Figure 1A, points l, 2, 3 and 4 ontrace T are level with the corresponding pointsv on trace S; the line product of the force exertedfby the rant and the vertical distance between thecorresponding points on. trace R and trace.S-.. 1 i

(b) In Figure 2A,; points 1 on the traces S, T are level, points 2 and 3 on trace T are below the level of the corresponding points on trace S and point 4 on trace T is above the, level. of that of trace S (thus corresponding; pointson. traces. S, T are-w level and there is no moment .due. to the; air load in the operative position (point 1), nor at a position. between points 2 and3, whilein other positions the moment due to the air load is not .large I compared with that exerted by the ram) (c) In Figure 3A, the point I on trace T is below that on traces, the points 2 are approximatelylevel,and the points 3 and 4 on trace T are above those of trace S.

There is thus no moment due to the air load at the position of point 2, and it will be seen that in the other positions the moment arm of the air load is smaller than that of the ram; this is especially. the case near the operative position of the grid where the air load is greatest.

Thus, it will be clear that with each of the above arrangements substantial advantage is obtained by reason of the reduced operating loads required from the ram 11.

With the arrangement or Figure l the load on. the grid due to theairfiow is completely balanced in each position of the grid, and the ram has therefor only to overcome the frictional forces andthe weight of the grid, while with the arrangement of Figures 2 and 3 the load due to the airflow is largely balanced in each position of the grid, and may be arranged to be completely balanced in at least. one position. Further, with the arrangements of Figures 2 and 3 the unbalanced force may be arranged to assist the final movement of the grid portion both to the retracted position and to the operative position. The arrangements of Figures 2 and 3 also have the advantage over Figure 1 that the parts connecting the grid portion 10 to the fixed structure are not liable to be fouled by debris caught by' the grid portion 10. Also with the arrangement of the invention the moment arm of the load applied by the ram about the instantaneous centre of rotation is largest when the grid section presents the greatest area to the gas flow.

Locking devices are preferably provided to lock the grid portion in the operative andinoperative positions, and these may be associated with the ram piston 11a and actuated by the fiuid pressure which. acts on the ram 'isindicated at 30. The engine is accommodated within a suitably profiled fairing 31, the forward end of which has an inturned portion 31a to provide the mouth of the air intake duct and the leading end of the outer wall of the duct. The air intake duct is of annular form and an intake bullet 32 is provided centrally of the fairing 31 to define the radially-inner Wall of the air intake duct.

The after end of the inturned portion 314 of the fairing 31 is cut away as indicated at 33 to provide a number of housing recesses, one for each of the retractable guard grid. elements 34 of which in the present construction there are sixfor the complete annular passage.

The guard grid elements 34 are shown in their operative positions in Figures 4 and 5, and their inoperative position is indicated in chain lines in Figure 4.

The after end of the housing recesses for the guard grid elements 34 are closed by an annular diaphragm 35 carrying at its outer periphery a seal member 36 which co-operate with an inner surface of the fairing 31. The diaphragm 35 is conveniently bolted to an outwardlydirected bolting flange 30a at the inlet end of the compressor casing 30, and this flange 30a may also have secured to it an annular extension member 37 on which the guard grid elements 34 are mounted.

Referring now more especially to Figures 6 and 7, each guard grid element 34 has a frame 38 formed from metal strip andthe frame 38 comprises rectilinear side portions and arcuate inner and outer portions, andthere are a number of slats 39 of aerofoil cross-section secured to the frame 38, which slats extend parallel to the central radius of the guard grid elements 34. A pair of anglesection members 40, 41, are secured to the outer arcuate portion of the frame 38; each of the members 40, 41 has one flange welded to the outer arcuate portion of the frame 38 and has its other flange lying against the flange of the other angle-section member. The guard grid elements are also curved transversely so that they conform to the profile of the outer wall 31a of the duct when in the retracted position.

The frame 38 and slats 39 are carried by an elongated supporting member 42 by means of a series of threaded studs, one, 43 of which, is disposed centrally of the supporting member 42, and the others 44 of which are engaged in threaded holes in brackets 45 formed as integral parts of the supporting member 42 at its ends. When the frame 38 is secured to the supporting member 42, the supporting member extends substantially tangential to the outer arcuate portion of the frame 38. The ends of the supporting member 42 are bored to receive spindles '46 which are secured in the bores by pins 47 and project from the supporting member 42 beyond its ends.

The spindles 46 are the means by which the retractable guard grid elements 34 are connected with operating rams of which there are two for each guard grid element 34, one ram being disposed adjacent each end of the outer arcuate portion of the frame 38.

Each ram has a cylinder 48 which is bolted to a flange 37a on the annular extension member 37, and comprises a piston 49 working in the cylinder 48 and having a boss 4% projecting from one end. The appropriate spindle 46 extends through the wall of the ram cylinder 48 and engages rotatively in the boss 49a as will best be seen from Figure 8. It Will be appreciated that as the piston 49 moves backwards and forwards so the spindle 46 is carried with it, and there is, therefore, provided in the wall of the ram cylinder 43, a slot 48a to permit the travel of the spindle 46. The slot is closed off by means of a slide member 50 carried on the end of the spindle 46 and the slide 50 has a projection 50a running in the slot 48a to prevent it from turning about the spindle 46. In this arrangement the piston 49 also has a piston rod 49b which acts as a guide member for the piston. Pressure air connections 51 are provided, one to each end of each ram cylinder.

Each retractable guard grid element 34 is also connected with the stationary structure of the air intake through pivoted links 52 of peanut form, the radially outer ends of which are pivoted to lugs 53 projecting from the brackets 45 at the ends of the supporting member 42, of: the grid element, and the radially inner ends of which are pivoted to the ends of brackets 54 projecting downwardly from the cylinders of the rams 48. The end of the brackets 54 extend radially inwards to beyond the lugs 53 'and the-lugs 53 are disposed real-wards of the lines joining the pivotal axes of the connection between the links 52 and brackets 54 and the axes of the spindles 46. It will be apparent that by this arrangement the loads required to move the grid elements between the retracted and extended positions are minimised as described with reference to Figure 2A.

A butter-55 is provided in each recess 33 to limit the movement of the corresponding guard grid element 34 into the recess.

in the arrangement of Figure 2 what happens as the piston 11:: moves to the right, is that point 22 starts to move around a circle in a clockwise direction and so the point 22 moves away from the line of travel of the pivot point 12 as represented by the line 12a. Thus, the line joining the points 12 and 22 must also swing in a clockwise direction to compensate for this increased distance. The swinging motion continues until the general plane of the grid 10 is horizontal when of course in practice it comes up against the buffer 55 (see for Example Fig. 4) and the motion to the right of the piston 11a is stopped.

1111 the opposite motions of the parts occur and the travel to the left by the piston is limited by the grid reaching the position in which its free end abuts the centre bullet 32, as shown in Figure 4.

We claim:

1. A debris guard arrangement for a duct in which fluid flows comprising a grid element adapted to be movable between an operative position in which it lies across the duct and a retracted position in which it is substantially out of the path of fluid flowing in the duct, an operating ram adjacent one Wall of the duct and connected to a point in one edge of the grid element to move said point in substantially a straight line parallel to the direction of fluid flow through the duct, and means rotatively supporting said grid element to rotate about an axis which is spaced away from the plane of said grid element and is at right angles to the line of movement of said edge on movement of said edge by said ram, so adapted and arranged that in each position of the grid element the instantaneous centre about which the grid element is rotated lies on or adjacent the line of action of the load on the grid portion due to the airflow in the air intake duct.

2. A debris guard arrangement for an annular air intake duct having an inner wall and an outer wall, comprising a plurality of guard grid elements adapted to extend in their operative position between the inner and outer walls of the annular duct and together to form an annular screen, a plurality of ram devices mounted adjacent the outer duct wall, there being at least one ram device for each grid element, each ram device having a piston rod pivotally connected to a point on an edge of the associated guard grid element adjacent the outer duct wall, the piston rod being adapted to move in a substantially straight line parallel to the outer duct wall, a plurality of brackets mounted on fixed structure adjacent the outer duct wall, one at each end of each guard grid element and projecting inwards from and at right angles to the tangent to the outer duct wall adjacent the centre of the grid, a pair of lugs projecting inwardly and rearwardly from the edge of each grid element adjacent the outer duct wall, one adjacent either end thereof, and a pair of links connecting each pair of said lugs and said brackets, whereby on operation of said ram to move the outer edge of the grid element in said substantially straight line the element is caused by said links to pivot about an instantaneous centre which lies on or adjacent the line of action of the load on the grid portion due to the airflow in the air intake duct.

3. A debris guard arrangement for a duct having fixed structure and in which fluid flows in a given direction, of

I the kind including a grid portion which is movable between an operative position in which it extends across the duct thereby to catch debris entrained in the fluid and an inoperative position in which the grid portion is substantially outside the path of the fluid through the duct, said grid portion having a centre of pressure, comprising first support means to which said grid portion is pivoted about a first axis transverse to said direction of flow, first guide means to co-operate with said first support means to constrain said first axis to move along a straight-line path, second support means to which said grid portion is pivoted about a second axis parallel to said first axis, second guide means to co-operate with said second support means to constrain said second axis to move along a second path which at any instant is at an angle to said first path such that a line normal to said second path and through said second axis, a line normal to said first path and through said first axis, and a line parallel to said direction of flow and through said centre of pressure intersect at substantially a single point namely the instantaneous centre of rotation of said grid portion,

at least over the'major part of the movement of said grid portion between its operative and its inoperative positions,

tatnaa andpower. means connected to. said fixed. structure, and to one-.of. said". supporttmeans. to. move the. support means relative to the. fixed. structure. between positions. corresppnd'ingto the. operative. andinoperative. positions of the grid portion.

I 4. A debrisguard. arrangement as. claimed in claim 3, comprisinga plurality of saidgrid portions, each portion heingtormed. as a sector of an annulus, and the portions being. arranged when in. their operative positions together iOrCXtEBd acrossthewhole. of an. annular flow passage.

5. A debris guard arrangement as claimed in claim 4,

wherein the power means comprisesa plurality of motors, one. corresponding. to and! being connected to each grid portionto. move it.

6. A debris guard arrangement as claimed in. claim 4, wherein thepower means comprisesv for each grid pora. corresponding pain of. operating rams connected to move: the grid portions, the rams being connected to the grid. portion atfcircumferentially' spaced points thereon.

7t, Atderbis. guard arrangement as claimed in claim. 4, comprising for each grid portion a corresponding pair of links which are pivotally connected at one end with the stationary structure at circumferentially spaced points and at their other ends to the. grid. portion.

8. A debris guard arrangement for a duct having fixed structure and in whichfluid flows in a given direction, of the ki'nd includinga grid portion which is movable between an operative position in whi'ch it extends across the duct thereby: to catch debris entrained in the fluidand an inoperative position in which the grid portion is substantially outside the path of the fluid through the duct, said grid portion having a centre of pressure, comprising boss means to which said grid portion is pivoted about a first axis transverse to said direction of flow, guide means parallel to the direction of flow through the duct in which said boss means is constrained to move along a straightline path, link means to which said grid portion is pivoted about a second axis parallel to said first axis at a point on said grid portion between said centre of pressure and said boss means, pivotal mounting means in fixedstructure of the duct at a point substantially aligned with said guide means said pivotal mounting means cooperating with said link means at a point thereon spaced from said second axis to permit pivoting of said link about the axis of said pivotal means and thereby to constrain said second axis to move along an are about said pivotal axis, the tangent to which at any instant is at an angle to said first path such that a line normal to said tangent and through said second axis, a line normal to said first path and through said first axis, and a line parallel to said direction of how and through said centre of pressure in tersect at substantially a single point namely the instantaneous centre of rotation of the grid portion, at least over the major part of the movement of said grid portion between its operative and its inoperative positions, and power means connected to said fixed structure and to said boss means to move the boss means relative to the fixed structure between positions corresponding to the operative and inoperative positions of the grid portion.

9. A derbis guard arrangement as claimed in claim 8, wherein the power means comprises a ram connected to the grid portion at its edge, and the link means comprises at least one link, the points of connection of each link and ram to the grid portion and to the fixed structure being geometrically such that, when a grid portion is viewed along its instantaneous axis of rotation, each link appears to have a length equal to half the distance between the centre of pressure of the grid portion and the point of connection to it of the ram, each link appears to be connected to the grid portion midway between its centre of pressure and its point of connection to the ram, and each link appears to be connected to the fixed structure on the line of travel of the point of connection between the grid portion to the ram.

10. A debris guard arrangement as claimed in claim 8,

being formed asa, sector of an annulusjand, theporltions being arranged when in their operativevv positions, together to extendkacross the. whole. of an annular flow passage.

11;. A'debris guard arrangement as claimed in claim 9, wherein the. power'means comprises a plurality of motors, one corresponding. to and being, connected to each grid portion to move, it.

12. A debrisguard' arrangement as claimed incl'aim 9, wherein the power means. comprises for each. grid ,portion a. correspondingpair of operating ramsconnected to move the. grid? portions, the rams bei'ngconnectedj. to the. gridportion. at. circumferentially spaced. points, there on. a

1.3.. A; debrisguard arrangement as claimed in. claim) comprising for each grid portion a. correspondingpair of said links. which are; pivotally connected at one, end with. the stationary structure at. circumferentiall'yl spaced points. and. at their other. ends to thegridportion.

14. A debris guard arrangement for aduct having fixed structure and in which fluidflowsv in a. given direction, of the kind including, a. grid. portionwhich is movable between: an, operative positionin which it extends across the duct, therebyv to catch. debris entrained inthe. fluid and an inoperative, position in which. the grid portion: is substantially. outside. the path. of the fluid through the duct, said gridportionhaving a. centre of pressure,.c.omprising bossmeans. to. whichL said, grid portion. is pivoted about a first. axis. transverse to said direction of. How, guide means parallel to the direction of airflow through the duct in which said boss means is constrained to move along a straight-line path, link means, pivotal mounting means in fixed structure on the downstream side of the grid portion and inwardly of the pivotal connection of the boss means to the grid portion said pivotal mounting means co-operating with said link means to permit pivoting of said link about the axis of said pivotal means, said grid portion being pivoted to said link means about a second axis parallel to said first axis at a point on said grid portion offset on the downstream side of a line joining said pivotal axis and said first axis and between these axes, whereby said second axis is constrained to move in an are about said pivotal axis the tangent to which at any instant is at an angle to said first path such that a line normal to said tangent and through said second axis, a line normal to said first path and through said first axis, and a line parallel to said direction of flow and through said centre of pressure intersect at substantially a single point namely the instantaneous centre of rotation of the grid portion, at least over the major part of the movement of said grid portion between its operative and its inoperative positions, and power means connected to said fixed structure and to said boss means to move the boss means relative to the duct between positions corresponding to the operative and inoperative positions of the grid portion. I

15. A debris guard arrangement as claimed in claim -14, comprising a plurality of said grid portions, each portion being formed as a sector of an annulus, and the portions being arranged when in their operative positions together to extend across the whole of an annular flow passage.

16. A debris guard arrangement as claimed in claim 15, wherein the power means comprises a plurality of motors, one corresponding to and being connected to each grid portion to move it.

17. A debris guard arrangement as claimed in claim 15, wherein the power means comprises for each grid portion a corresponding pair of operating rams connected to move the grid portions, the rams being connected to the grid portion at circumferentially spaced points thereon.

18. A debris guard arrangement as claimed in claim 15, comprising for each grid portion a corresponding pair of said links which are pivotally connected atone end with the stationary structure at circumferentially spaced points and at their other ends to the grid portion.

19. A debris guard arrangement for a duct having fixed structure and in which fluid flows in a given direction of the kind including a grid portion which is movable between an operative position in which it extends across the duct thereby to catch debris entrainedin the fluid and an inoperative position in which the grid portion is substantially outside the path of the fiuid through the duct, said grid portion having a centre of pressure, comprising first boss means to which said grid portion is piv-- oted about a first axis transverse to said direction of flow, first guide means parallel to the direction of flow through the duct in which said first boss means is constrained to move along a straight-line path, second boss means to which said grid portion is pivoted about a second axis parallel to said first axis, said second boss means cooperating with a lug on said grid portion extending on the downstream side thereof and inwardly of said first axis,

' second guide means which diverge from said first guide 12 portion between its operative and its inoperative positions, and power means connected to said fixed structure and to one of said boss means to move the boss means relative to the fixed structure between positions corresponding to the operative and inoperative positions of the grid portion.

,20. A debris guard arrangement as claimed in claim 19, comprising a plurality of said grid portions, each portion being formed as a sector of an annulus, and the portions being arranged when in their operative positions together to extend across the whole of an annular flow passage.

21. A debris guard arrangement as claimed in claim 20, wherein the power means comprises a plurality of motors, one corresponding to and being connected to each grid portion to move it.

22. A debris guard arrangement as claimed in claim 20, wherein the power means comprises for each grid portion a corresponding pair of operating rams connected to move the grid portions, the rams being connected to the grid portion at circumferentially spaced points there References Cited in the file of this patent UNITED STATES PATENTS 2,546,153 De Remer Mar. 27, 1951 2,555,592 Kelly June 5, 1951 2,618,358 Newcomb Nov. 18, 1952 2,704,136 Rainbow Mar. 15, 1955 

